This invention relates to a surface profiler, more particularly a surface profiler using a "flying spot".
The use of various kinds of industrial vision systems has become quite widespread. Such systems find common application in robotic systems, such as robotic welding systems wherein the industrial vision systems or devices are used to control a welding head such that it follows a weld seam. Industrial vision systems are used for numerous other applications such as generating data on the profile of a surface, which data can be conveniently stored electronically. Such industrial vision sensors have included systems using TV cameras, linear detector arrays, and flying spot camera systems using lasers.
Although impressive improvements have been made in vision sensors using TV cameras, a number of disadvantages remain. In particular, the sensitivity, dynamic range, and background light rejection capabilities of TV cameras have posed significant limitations on their use for industrial vision sensing. Considering that the directional reflectivity of angled metal surfaces with typical industrial finishes can easily vary over factors of several thousand, the dynamic range on the order of several hundred to one of standard TV camera detector systems will be greatly exceeded. Additionally, the limited sensitivity of TV cameras typically requires a relatively high power laser which must be carefully controlled to avoid posing safety hazards.
Various structured light profile determination systems can be used to generate a pattern of different wavelength light to be applied to a surface. For example, by sensing the reflected light from the pattern by use of two detector arrays (one for each of the two light wavelengths), one can develop a profile mapping of the surface. U.S. Pat. No. 4,349,271 issued on Sept. 14, 1982 to Joseph L. Mundy, Gilbert B. Porter III, and Thomas M. Cipolla, entitled "NON-CONTACT MEASUREMENT OF SURFACE PROFILE", and assigned to the assignee of the subject invention, discloses such a structured light profile determination system.
Although such structured light profile determination systems using arrays are generally useful, they are subject to several disadvantages. In particular, the resolution of such systems is limited by the density of the detector elements in the detector array. Additionally, the requirement for a large number of detector elements to provide reasonable resolution and a sufficient field of view tends to limit the quality of the detector element which may be used. That is, a detector element such as a photomultiplier tube which is highly sensitive and has a relatively good dynamic range is too expensive for use in an array for many structured light applications. On the other hand, a detector element such as a photodiode which is less expensive can be used to realize a relatively low priced array, but lacks the sensitivity and dynamic range of a photomultiplier tube.
Prior art flying spot profile measuring systems have typically used a laser beam which is scanned onto a surface. The image of the beam hitting the surface is tracked by a detector array which is offset from the angle at which the beam is directed to the surface. Using optical triangulation techniques, knowledge of the angle at which the beam strikes the surface and of the position of the beam's image on the surface allows determination of the surface profile. Because such systems have generally used detector arrays, such systems are also subject to sensitivity, dynamic range, and resolution limitations found in the detector arrays used in structured light systems. Additionally, the variations in surface directional reflectivity may require the use of a relatively powerful laser necessitating various special safety precautions which are economically disadvantageous and operationally cumbersome.
Another form of profile measuring system, described in U.S. Pat. No. 4,158,507 employs a laser beam which is scanned across a surface and has its image detected by a single photomultiplier tube. In order to trace the path of the scanned beam across the surface, an optical grating is disposed between the image of the beam and the photomultiplier. As the beam sweeps across the surface, the sweep time between various transparent strips on the grating is indicative of the slope of the surface. Such a system suffers from limited resolution. In particular, if the grating is quite fine (very narrow alternating transparent and opaque strips), the system will be able to detect small variations in surface slope. However, such very narrow grating strips could produce erroneous or misleading detection patterns. Specifically, if there is a sharp surface change or major vertical step in the surface, a beam's image may jump over more than one strip of the grating such that the detection system produces an erroneous indication of the height of that step.
A flying spot profiler using only two sensors is disclosed in co-pending U.S. patent application Ser. No. 714,416 of C. Murray Penney, filed Mar. 21, 1985 entitled "METHOD AND SYSTEM FOR DETERMINING SURFACE PROFILE INFORMATION", assigned to the assignee of the present application and hereby incorporated by reference. That invention uses a feedback arrangement to vary the angle at which a laser beam is applied to the surface under test such that the image of the beam on the surface tends to be maintained constant with respect to a linear reference position. The variations in the angle of application of the laser beam is indicative of the surface height. The arrangement avoids the position uncertainty present in the above-noted optical grating profiler.